The present invention relates to a method and apparatus for cooling sheet steel or the like with water sprays.
In general, the cooling treatments of sheet metal must be accomplished in normalizing treatments and after hot rolling. The principal objectives in normalizing treatments are grain refinement, stress relief and chemical homonization, whereby desired mechanical strength such as ductility may be obtained. The hot-rolled sheet metal must be cooled to a uniform temperature without being quenched so that the succeeding processing of hot-rolled sheet metal may be facilitated and consequently high productivity may be attained.
In the case of normalizing process, the sheet metal is heated above an austenitic transformation temperature and then is cooled. If the cooling rate can be varied within the range in which quenching will not result; that is, if the cooling rate may be optimumly selected depending upon the composition of the sheet metal to be subjected to the normalizing treatment, the back cooling table can be made compact in size and, furthermore, the safeguarded operation may be ensured because the time period for leaving the high-temperature sheet steel on the table may be considerably shortened.
In the cooling treatment both in the normalizing process and after the hot rolling process, care must be taken to eliminate the danger of distorting the shape of the sheet.
The methods for cooling the high-temperature sheet steel may be divided into the air cooling, forced cooling and water cooling methods as will be described in detail below. Recently there has been devised and demonstrated a cooling method in which the water is mixed with a suitable atomizing agent such as air or steam and sprayed over the surfaces of the hot sheet steel.
(i) Air Cooling:
Hot sheets are left on the cooling stand or table and are cooled with air. The thermal conductivity of air is however on the order of 80 kcal/m.sup.2 .multidot.hr.multidot..degree.C. so that the cooling rate is very slow. As a result, the cooling stands or tables considerably large in size must be used. In addition, by air cooling the grain refinement is limited.
(ii) Forced Air Colling:
This is the method in which a large amount of air is forcibly blown against the surfaces of the hot sheet steel with the use of a blower or the like. However, the thermal conductivity between the sheet steel and the air is on the order of 100 kcal/m.sup.2 .multidot.hr.multidot..degree.C. at the highest so that the same problems as encountered in the air cooling arise.
(iii) Water Spray Cooling:
The water is sprayed through the nozzles of metal pipes against the surfaces of the hot sheet steel. This method has been so far used for hardening treatments. However, the prior art water spraying apparatus cannot attain the complete atomization of water so that the water drops and jets are impinged against the surfaces of the sheet steel. As a result, it is difficult to reduce the thermal conductivity between the sheet steel and cooling water below 1,000 kcal/m.sup.2 .multidot.hr.multidot..degree.C. Furthermore, the portions impinged with water droplets are quenched or hardened. In addition, since the water spray cooling apparatus has been used for quenching, cooling water must be sprayed at a high flow rate of from 500 to 5,000 l/m.sup.2 .multidot.min. With the prior art water spraying apparatus, it is very difficult to control the water spray rate to less than 100 l/m.sup.2 .multidot.min.
(iv) Water Spray Cooling with Atomizing Agent such as Air or Vapor:
This method has been recently devised and is advantageous in that the cooling rate may be varied over a wide range, but disadvantageous in that a large quantity of atomizing agent is required and the additional energy for atomizing water must be provided. That is, when the air is used as an atomizing agent, the air of from 300 to 400 l is required for atomizing 1 l of water. This means that the energy of 2,500 W must be supplied to a compressor so as to raise the pressure of air of 300 l to 6 kg/cm.sup.2 in order to atomize 1 l of water per minute. On the other hand, according to the present invention, only 50 W is supplied to a pump in order to obtain the nozzle pressure of 10 kg/cm.sup.2. That is, the power consumption of the prior art water spraying method is as high as 50 times. Furthermore, the prior art water spraying method presents the noise problem because when the atomizing agent flows through the nozzles at high velocities, noise as high as from 90 to 110 dB is produced.
The above-described water spray cooling methods (iii) and (iv) have some common problems to be described below.
In the sheet steel production line, the sheets are transported over the horizontal tables or the like from one station to another. As a result, the cooling apparatus must be horizontal; that is, the cooling water is sprayed vertically against the upper and lower surfaces of the sheet. In this case, the water spread against the lower surface of the sheet drops therefrom by gravity so that no problem arises, but the cooling water sprayed over the upper surface remains there, forming heat-insulating layers against the cooling water. As a result, the cooling water must absorb the heat from the upper surface of the sheet through these layers of remaining water so that the effective cooling cannot attained. Furthermore, the remaining cooling water will not form a layer in uniform thickness over the whole surface so that the uniform cooling of the upper surface is impossible.
In the case of water spray cooling of the sheet steel which has been heated to the temperatures higher than 100.degree. C., layers or films of vapor are formed between the cooling water and the sheet steel. This vapor film formation is different between the upper and lower surfaces mainly because of the difference in amount of water remaining on the upper and lower surfaces. Furthermore, vapor layer or film formation also differs even over the same surface because of the non-uniform temperature distribution. As a result, local heat transfer rates between the cooling water and the sheet metal vary almost from one point to another. Thus, because of the non-uniform cooling with the resultant non-uniform local heat transfer rates, distortion of the sheet metal results. Furthermore, distortion causes the change in pattern of cooling water remaining over the upper surface so that the cooling conditions change or become worse. As a consequence, the cooled sheet steel cannot have the uniform structure. That is, the production of sheet steel with high qualities cannot be attained. So far the distorted sheet steel has been straightened or corrected by a leveler or the like.
The present invention was made to overcome the above and other problems encountered when the sheet metal or steel which has been heated to high temperatures is cooled with water sprays.
The primary object of the present invention is therefore to provide a method and apparatus in which an optimum coefficient of heat transfer from sheet steel to cooling water may be obtained by controlling the transportation speed of sheet steel, the water spraying rate (that is, the rate at which the cooling water is sprayed), the pressure of cooling water at nozzles and the spray angle, whereby the sheet steel may be uniformly cooled without causing distortion of the shape.
The above and other objects, features and advantages of the present invention will become more apparent from the following description of a preferred embodiment thereof taken in conjunction with the accompanying drawings.